1. Field of the Invention
The present invention relates to a semiconductor laser device which may suitably be applied to an electrophotographic color reproducing machine, a laser printer or the like.
2. Description of the Prior Art
One type of electrophotographic color reproducing machine or other similar means employs a semiconductor laser as a means for forming an electrostatic latent image on a photosensitive image retainer using an image signal corresponding to a document.
FIG. 6 shows one example of a simple type color reproducing machine 10. This machine is arranged to record a color image of a color document by separating color data concerning the document into a number of different kinds of color data which is on the order of three. As color data items which are to be separated from each other, three colors, that is, black BK, red R and blue B, are shown by way of example.
In FIG. 6, the reference numeral 11 denotes an image retainer in the shape of a drum which has a photoconductive photosensitive surface layer such as selenium formed on its surface so that an electrostatic image (electrostatic latent image) corresponding to an optical image can be formed on the surface of the image retainer 11.
Members which will be explained below are successively disposed along the peripheral surface of the image retainer 11 in the hereinafter mentioned order in the direction of rotation of the image retainer 11.
The surface of the image retainer 11 is electrically charged uniformly by the operation of a charging device 12. The charged surface of the image retainer 11 is exposed to light (denoted by the numeral 14) to form an optical image based on each color separation image.
The electrostatic latent image formed on the surface of the image retainer 11 by the exposure is developed by means of predetermined developing devices. A number of developing devices which corresponds to the number of color separation images are disposed adjacent image retainer 11. In this example, a developing device 15 charged with a developer for red toner, a developing device 16 charged with a developer for blue toner and a developing device 17 charged with a developer for black toner are successively disposed in that order in the direction of rotation of the image retainer 11 in such a manner that these developing devices face the surface of the image retainer 11.
The developing devices 15 to 17 are sequentially selected in synchronism with the rotation of the image retainer 11, and when, for example, the developing device 17 is selected, a black color separation image is developed.
A pre-transfer charging device 19 and a pre-transfer exposing lamp 20 are provided at the downstream side of the developing device 17 in terms of the direction of rotation of the image retainer 11 to facilitate transfer of a color image to a recording medium P. However, the pre-transfer charging device 19 and the pre-transfer exposing lamp 20 may be provided according to need.
The color image developed on the surface of the image retainer 11 is transferred to the recording medium P by the operation of a transfer device 21.
The recording medium P having the transferred color image is subjected to a fixing process carried out by a fixing device 22 which is provided at the downstream side of the transfer device 21, and the recording medium P is then discharged.
A charge eliminating device 23 which may be employed according to need comprises of either one or both of a charge eliminating lamp and a charge eliminating corona discharger.
A cleaning device 24 comprises of a cleaning blade, a fur brush, etc. and is adapted to remove with these components remaining toner attached to the surface of the drum constituting the image retainer 11 after the color image has been transferred therefrom.
As the above-described charging device 12, it is possible to employ, for example, a Scorotron Corona discharger. This type of charging device is less affected by the previous charge and therefore is capable of giving a stable electric charge to the surface of the image retainer 11.
As the light for exposure, light which is obtained from a laser beam scanner is used in this example.
In the case of a laser beam scanner, it is advantageously possible to use a compact and inexpensive semiconductor laser as a light source of the image recording machine and also to record a clear color image.
An image exposing means which is shown in FIG. 7 illustrates one example of the laser beam scanner 30.
The laser beam scanner 30 has a semiconductor laser 31 which is subjected to optical modulation on the basis of color separation images (e.g., binary data).
The laser beam which is emitted from the laser 31 is passed through a collimator lens 32 and a cylindrical lens 33 and made incident on a mirror scanner 34 which comprises a rotary polygon mirror.
The laser beam is deflected by the mirror scanner 34 and applied to the surface of the image retainer 11 through an f-.theta. lens 35 and a cylindrical lens 36 for image formation.
The mirror scanner 34 causes the laser beam to scan across the surface of the image retainer 11 at a constant speed and in a predetermined direction a, and exposure in correspondence with each color separation image is effected by this scanning.
It should be noted that the reference numeral 39 denotes a photosensor which receives the laser beam reflected from a mirror 38 to obtain an index signal which indicates the start of a laser beam scanning operation, and image data for one line is written on the basis of this index signal.
Employment of the laser beam scanner 30 enables facilitation of an electrostatic image forming operation in which an electrostatic image is formed while being displaced for each color separation image, and it is therefore possible to form a clear color image.
FIG. 8 is a circuit diagram of one example of a laser driving circuit 40.
The laser driving circuit 40 is provided with a circuit for driving a laser with a modulation signal, together with a light quantity stabilizing circuit for stabilizing the quantity of light emitted from the laser.
In FIG. 8, a current bypass transistor 42 is connected in parallel to the laser 31, and a modulation signal based on image data supplied to a terminal 43 is supplied to the transistor 42 through a driver 44. Thus, the transistor 42 is ON/OFF controlled in response to the modulation signal, and the supply of the current to the laser 31 is thereby controlled.
A variable constant-current circuit 45 is connected to an intermediate portion of the current path of the laser 31 to control the quantity of current supplied to the laser 31 in accordance with the quantity of light emitted from the laser 31.
For this purpose, the quantity of light emitted from the laser 31 is detected by means of a photosensor 50, and the detected current is converted in a current-to-voltage converter 51 into a voltage signal proportional to the detected current, that is, the quantity of emitted light. Thus, the photosensor 50 and the current-to-voltage converter 51 constitute in combination a light quantity monitor circuit 52.
The voltage signal is supplied to a control circuit 53 where a control signal the level of which corresponds to the level of the voltage signal is generated. The variable constant-current circuit 45 is controlled by the output of the control circuit 53. The constant-current circuit 45 comprises a differential amplifier 46 and a transistor 47.
Accordingly, when the quantity of light emitted from the laser 31 exceeds that in a normal operation, the control signal obtained from the control circuit 53 decreases, and the driving current flowing through the laser 31 decreases correspondingly, so that the quantity of light emitted from the laser 31 is reduced.
In the above-described arrangement the laser 31 is controlled in accordance with the output of the control circuit 53 so that the driving current supplied to the laser 31 is maintained at a constant level. However the laser 31 has disadvantageous temperature characteristics, so that the quantity of light emitted therefrom varies with changes in the ambient temperature.
Further, in the case where the laser 31 is subjected to light quantity feedback control (not shown), there is a fear of the quantity of emitted light becoming excessively large so as to lead to deterioration of the laser 31.
There is another means which is arranged to monitor the driving current flowing through the laser 31 and suspend the drive of the laser 31 when the current value exceeds a predetermined value. This means, however, cannot satisfactorily protect the laser 31 at all times if there are variations in characteristics of the laser 31 in terms of the relationship between the driving current and the quantity of emitted light.
It is a matter of course that the laser 31 cannot satisfactorily be protected even in the case of employment of a means which is arranged to suspend the drive of the laser 31 when the ambient temperature changes and the driving current exceeds a predetermined value.
FIG. 9 is a circuit diagram of another example of the laser driving circuit 40.
This laser driving circuit 40 is provided with a light quantity stabilizing circuit for stabilizing the quantity of light emitted from the laser in addition to a circuit for driving the laser in response to a described example.
The reason for the provision of the light quantity stabilizing circuit is that the temperature characteristics of lasers are exceedingly inferior and therefore, when the use of a laser in the environment where the ambient temperature may change is taken into consideration, there is a need for a light quantity stabilizing circuit for stabilizing the quantity of light emitted from the laser.
The laser 31 is excited by means of a driving current (exciting current) output from a current generating circuit 59 to emit a quantity of light which corresponds to the level of the driving current. The laser beam emitted from the laser 31 is received by a photosensor 50 to detect the quantity of light emitted from the laser 31, and a current corresponding to the detected light quantity is supplied to a current-to-voltage converter 51 where the current is converted into a voltage signal corresponding to the detected light quantity. Accordingly, the photosensor 50 and the current-to-voltage converter 51 cooperate with each other to function as a light quantity monitor circuit 52.
The voltage signal is supplied to a voltage comparing circuit 54 where it is compared with a reference voltage delivered from a reference voltage source 62, and the output of the voltage comparing circuit 54 is supplied as an up/down control signal to an up/down counter 56.
The counter 56 is arranged to count clocks of a predetermined frequency which are supplied from an oscillator 57. The counter 56 is supplied at a clear terminal CLR thereof with a clear signal Sc which is delivered from a microprocessor (microcomputer) which controls the reproducing machine, to clear the counter 56. The counter 56 is further supplied at an enable terminal EN thereof with a count enable signal Sh.
The digital output from the counter 56 is converted into an analog signal in a D/A converter 58 which is connected to the output side of the counter 56, and this analog signal is supplied as a current control signal to a current generating circuit 59.
When the count enable signal Sh is available after the counter 56 has been cleared in response to the clear signal Sg, the current generating circuit 59 is supplied with a control signal (a high-level signal) in place of the modulation signal from the microcomputer through an OR circuit 55. At the same time, the count enable signal Sh activates the counter 56 to start a count-up operation, and as the count-up operation proceeds, the driving current supplied to the laser 31 increases gradually. When the laser 31 is excited, the quantity of laser light also increases gradually.
Until the laser light quantity reaches a predetermined level, the output of the voltage comparing circuit 54 is maintained at a high level, and the counter 56 is thus allowed to continue the count-up operation. When the laser light quantity reaches a predetermined level, the output of the voltage comparing circuit 54 is inverted to a low level, thus causing the counter 56 to be shifted to the down-count mode, and as the count-down operation of the counter 56 proceeds, the laser light quantity decreases gradually contrary to the above. However, when the laser light quantity lowers to a level at which the output of the voltage comparing circuit 54 is inverted, the operation mode of the counter 56 is inverted to the up-count mode.
Accordingly, ff the control is effected so that the count enable signal Sh is turned OFF when a predetermined period of time has elapsed, then the counter 56 holds an output level immediately before the signal Sh is turned off, so that the laser 31 is excited with a constant driving current at all times and the quantity of light emitted from the laser 31 is maintained at a constant level at all times.
Thus, the laser 31 is modulated on the basis of a modulation signal supplied to the terminal 43 in a state wherein the quantity of light emitted from the laser 31 is controlled so as to be maintained at a constant level, and an electrostatic latent image corresponding to image data concerning a particular document is formed on the drum 11 which serves as an image retainer.
FIG. 10 shows the relationship between the driving current supplied to the laser 31 and the quantity of light emitted therefrom. As will be clear from the graph shown in FIG. 10, the quantity of emitted light increases in extremely minute amounts until the driving current reaches a predetermined value Ith. However, when the driving current exceeds Ith, the light quantity increases rapidly.
In the light quantity stabilizing circuit shown in FIG. 9, the counter 56 performs either a count-up or count-down operation in which the count is incremented or decremented by one, and therefore, it will take considerably long time for the driving current to reach the above-described point of inflection Ith when it increases from zero.
The control operation for holding the light quantity at a constant level is generally conducted as one step in a pre-processing (a warming-up processing) which is carried out before printing for one page is executed. Therefore, when the stabilization of laser light quantity takes much time, the output of an image to be recorded is delayed and the operator must stand by for an undesirably long time.